I just finished installing gentoo on a SparcServer 670MP. Upon reboot I'm getting the famous "file does not appear to be executable" error message.
Its worth noting that my /boot directory is located on my / partition; I have no seperate /boot partition.

One problem may be that the whole disk partition needs to be at partition 3 on the drive to be booted. I noticed your are at partition 1, which may be causing the problem. You can try boot disk:a or boot disk:1 and see if those help.

I had exactly the same problem, but I was trying to dual boot solaris and gentoo. I had set it all up as per the Sun pages, and set partition 7 as root and 8 as swap for gentoo, and 1 for root and 2 for swap for solaris, and 3 for the whole disk.

Needless to say it didnt work, I kept seeing what you are seeing even though I had set an nvram alias in OBP.

My solution was to use SuSe. Install this and at the end of the SILO setup when you select expert, you can set it to boot and write an nvram alias, then just boot from Gentoo live CD format partitions and start again.

Works for me, and as noone else has posted a decent answer for either you or me, I would give it a go!